(a) Field of the Invention
The present invention relates to an optical waveguide including multiple cores in a clad layer, a method of manufacturing the same, and an optical waveguide mounted board.
(b) Description of the Related Art
In recent years, optical waveguides capable of high speed data transfer have been used even in short-distance communications, such as ones within a wiring board and between wiring boards.
As a prior art optical waveguide, there is known an optical waveguide having a multi-layered arrangement in which multiple cores are stacked together (e.g., Japanese Patent Application Publication No. 2007-293244, hereinafter referred to as Patent Document 1). This optical waveguide having a multi-layered arrangement is manufactured as follows. First, a first clad layer and then a first core layer are formed on a substrate. Then, the first core layer is exposed to a pattern of light and developed to form multiple first layer cores. Next, a second clad layer is formed which covers the first layer cores and the first clad layer. After that, a second core layer is formed on the second clad layer. The second core layer is then exposed to a pattern of light and developed to form second layer cores. Further, a third clad layer is formed on the second layer cores and the second clad layer. Thereafter, the same steps are repeated to form an optical waveguide having a multi-layer structure.
However, in the prior art optical waveguides, it is always necessary to form a clad layer between different core layers in the process of stacking cores together. This is because cores protrude from a clad layer when the cores are formed on the clad layer, and therefore the cores and the clad layer need to be covered with a clad layer and planarized before the next core is formed. For this reason, the size of the optical waveguide in the thickness direction increases, resulting in a decrease in the wiring density of optical wiring.
On the other hand, there are needs for the following optical waveguides: an optical waveguide which includes optical wiring having cores with different core diameters, and providing different transfer speeds depending on the core diameters; and an optical waveguide in which mirrors are formed at different positions and angles on different cores to allow light receiving/emitting elements to be mounted on both sides of the optical waveguide. However, in the aforementioned prior art method of manufacturing an optical waveguide, it is impossible to form cores having different thicknesses in a single layer, or to form cores having mirrors formed at different positions and angles in a single layer. For this reason, in the prior art optical waveguides, in order to form cores having different core diameters or form cores each having mirrors formed at different positions and angles, multiple cores having different core diameters or different mirror positions need to be stacked in the thickness direction of the optical waveguides. This increases the sizes of the optical waveguides in the thickness direction.